1) Field Of The Invention
The field of this invention relates to a unique, simple and cost effective technique for improving the linearity of a power amplifier for banded applications such as are found in mobile and cellular and microwave communication bands.
2) Description Of The Prior Art
The Federal Communications Commission (FCC) has stringent requirements for mobile and cellular communications. One of these requirements is that the communication circuit must be highly linear. The more linear the system, the less noise is created. The less noise, the higher the quality of the cellular signal. Also, the possibility of interference between signals is decreased.
By using Code Division Multiple Access (CDMA) and Time Division Multiple Access (TDMA), channel capacity can be increased significantly. Let it be assumed that one is able to obtain five hundred channels within every two megahertz (MHz) of output of a power amplifier. By using CDMA and TDMA the channel capacity can be increased by approximately a factor of ten. Therefore, instead of getting only five hundred channels with two MHz there can be obtained five thousand channels. With the increase in the number of channels, there is the need for better linearity.
In order to achieve reasonable efficiency from a power amplifier, a mode of operation is preferred which exhibits non-linear behavior. To obtain this efficiency in the past there have been various power amplifier linearization techniques that have been utilized to meet the requirements of the Federal Communications Commission and have a desirable level of power. These prior linearization techniques have utilized feedback of the output signal into the input side of the power amplifier.
There are two types of feedback, regenerative and degenerative. Regenerative feedback is called positive feedback and degenerative feedback is called negative feedback. The positive feedback signal increases the amplitude of the original signal while the negative feedback signal decreases the amplitude of the original signal. In positive feedback, the voltage output of the amplifier is increased because the effective input voltage is increased. This greater amplification also usually increases the amount of distortion and noise in the amplifier. Additionally, in positive feedback there is produced sustained oscillations. Because of the problems associated with positive feedback, it is common to utilize negative feedback. In negative feedback, the voltage output of the amplifier is decreased because the effective input voltage is decreased. This type of feedback is used to reduce the effects of distortion, hence improving linearization. The negative feedback improves the frequency response and stability of amplifiers.
Negative feedback techniques for power amplifiers have, in the past, improved linearization to some extent. However, in current mobile and cellular communications, the requirements are becoming quite stringent for achieving the best possible quality of a narrow band signal so that the greatest number of channels can be utilized within the smallest possible range of power. In the past it has not been known to utilize a linearization technique for a power amplifier that will achieve a substantial increase in channel capacity within a given range of power.